1. Field of Invention
The present invention relates to a current-controlling apparatus, and more particularly, to a current-controlling apparatus using a feedback control to adjust the current passing through a light emitting diode string (LED string) for adjusting the brightness of the LED string.
2. Description of the Related Art
For a backlight source implemented in LED mode of a liquid crystal display television (LCD television), a large number of LEDs are employed to make the backlight source match a cold cathode fluorescent lamp (CCFL) in terms of the brightness thereof. In order to reduce the number of the driving integrated circuits (driving IC) for the LEDs and lower the total driving current of the LEDs, the circuit of the backlight source is usually designed by employing multiple LEDs in series connection for lightening the same. Such a design not only reduces the set number of the driving ICs, but also lowers the total driving current of the LEDs and further lowers the consumption power of the driving ICs.
However, it is difficult to make the cut-in voltage (standing for the lowest voltage to turn on an LED) of every LED completely consistent with each other in an LED manufacturing process. Consequently, the error values for the cut-in voltage of every LED would be accumulated, which results in difference between the currents of each LED string set due to the inconsistent cut-in voltages under a constant input voltage. As a result, each of the individual LED string sets will have a different brightness. Therefore, a phenomenon of uneven brightness or uneven chrominance appears on the backlight source of a display panel.
To solve the above-mentioned problem, some of improvement schemes by using current mirrors were provided. In the U.S. Pat. No. 5,701,133, for example, a scheme is given by FIG. 1. FIG. 1 is a conventional brightness-adjusting circuit. Referring to FIG. 1, the symbol VLED represents a power voltage, GND represents a grounding voltage and Vin represents an input signal. The circuit shown by FIG. 1 is two current mirrors in series connection (102 and 103 in FIG. 1) formed by bipolar junction transistors (BJTs, for example, 101 in FIG. 1), respectively. Wherein, the current amount of the LED string 104 is controlled by taking the advantage that the current Im1 of the current mirror 102, the current Im2 of the current mirror 103 and the current Ic are equal to each other. In this way, the currents of every LED string set in a circuit with multiple sets of LED strings are controlled to be consistent with each other, thus the desired even brightness is achieved.
Note that the above-described circuit is a control system with an open loop by nature. Therefore, once an LED string in the system is malfunctioned (for example, some of LEDs in an LED string are short circuited), or an LED string has an excessive error of the total cut-in voltage (for example, the temperature characteristic of each LED slightly different from each other results in a larger error of the total cut-in voltage), the malfunction can not be detected due to lack of a feedback control mechanism. The BJTs of the current mirror may receive a great amount of voltage and currents, resulting in an overheat risk due to a constantly rising temperature thereof. Therefore, the reliability of products based on the above-described scheme is questionable.
Similarly, the U.S. Pat. No. 6,556,067 and No. 6,636,104 also employ current mirrors characterizing the same open loop control mode to make the currents of all LED string sets consistent with each other to achieve the brightness evenness. Thus, the reliability of such products is also in doubt.